1. Field of the Invention
The present invention relates to a multilayer printed wiring board and a printed wiring board that have a small dielectric loss to adapt to high-frequency signals, and a prepreg and a laminate such as a multilayer printed board that are used for manufacturing the wiring boards.
2. Background Art
In recent years, signal bands of information communication apparatuses such as PHS and cellular phones, and CPU clock times of computers have reached GHz bands and using higher frequencies has progressed. The transmission loss of electric signals is expressed by the sum of dielectric loss, conductor loss and radiation loss, and there is such a relation that a higher frequency of electric signals effects a higher dielectric loss, conductor loss and radiation loss. Since the transmission loss attenuates electric signals to damage the reliability of the electric signals, in printed wiring boards handling high-frequency signals, it is necessary to suppress increases in dielectric loss, conductor loss and radiation loss. The dielectric loss is proportional to the product of a square root of a relative permittivity of an insulator on which circuits are formed, a dielectric loss tangent, and a frequency of signals used. Therefore, selection of an insulating material having a low relative permittivity and dielectric loss tangent as an insulator can suppress an increase in dielectric loss.
Examples of typical resin materials having a low permittivity and a low dielectric loss tangent include fluororesins such as polytetrafluoroethylene (PTFE). Since fluororesins have both a low relative permittivity and a low dielectric loss tangent, they have been used for board materials handling high-frequency signals for a long time. By contrast, various types of non-fluoro insulating materials have been studied which can easily been made into varnishes by an organic solvent, and have a low molding temperature and curing temperature, easy handling, and a low permittivity and dielectric loss tangent. The non-fluoro insulating materials include an example described in JP Patent Publication (Kokai) No. 08-208856A (1996) in which a diene polymer such as polybutadiene is impregnated in a glass cloth and cured with a peroxide. They further include examples described in JP Patent Publication (Kokai) No. 11-124491A (1999) in which a cyanate ester, a dienic polymer and an epoxy resin are heated to be made into B-stage. They further include an example described in JP Patent Publication (Kokai) No. 09-246429A (1997) of a resin composition composed of an allylated polyphenylene ether, a triallyl isocyanate and the like, examples described in JP Patent Publication (Kokai) No. 2002-249531A and JP Patent Publication (Kokai) No. 2005-89691A in which polyfunctional styrene compounds wholly having hydrocarbon skeletons are used as crosslinking components, and many other examples.
On the other hand, insulating layers of printed wiring boards often contain a glass cloth as a base material; and various studies on making the base material into a low-permittivity one and a low-dielectric loss tangent one have been carried out. The examples include an NE glass cloth described in JP Patent Publication (Kokai) No. 9-74255A (1997) in which the formulation ratio of silicon oxide, aluminum oxide and boron oxide is prescribed, a glass cloth described in JP Patent Publication (Kokai) No. 2005-336695A in which a quartz glass fiber and a non-quartz glass fiber are composited, and a fine quartz glass fiber of 7 μm or less in diameter and a manufacturing method of a cloth using the fiber described in JP Patent Publication (Kokai) No. 2006-282401A. Compositing a resin composition and a glass cloth having low permittivities and low dielectric loss tangents can provide an insulating material for printed wiring boards excellent in dielectric properties, mechanical properties such as strength as well as thermal properties such as thermal expansion.